The invention relates to a metal-ceramic substrate with a multi-layer, plate-shaped ceramic material and comprising at least one metallization, metal lining, provided on or applied to the surface of the ceramic material, which metallization is bonded to the ceramic material by means of direct bonding (DCB) or active soldering. The ceramic material consists of at least one inner layer or base layer of a silicon nitride ceramic, and the surface of the ceramic material provided with the at least one metallization being formed from an intermediate layer of an oxidic ceramic applied to the at least one base layer.
The invention further relates to a method for manufacturing a metal-ceramic substrate comprising a multi-layer, plate-shaped ceramic material, which consists of at least one inner layer or base layer of a silicon nitride ceramic and comprising at least one metallization provided on a surface of the ceramic material, in which an intermediate layer of an oxidic ceramic is formed on the surface of the ceramic material on the at least one base layer to be provided with the at least one metallization and the at least one metallization is applied to said intermediate layer by direct bonding (DCB) or active soldering of at least one metal layer or foil.
Metal-ceramic substrates or ceramic substrates with metallizations are known in the art, particularly also as circuit boards or substrates for electrical and electronic circuits or modules, especially for high-power circuits or modules.
Also known is the so-called DCB process for the direct bonding with a ceramic material or substrate for manufacturing the metallization required for circuit boards, connections, etc. on a ceramic substrate, e.g. on an aluminum oxide ceramic substrate. In this method, described for example in US-PS 37 44 120 or DE-PS 23 19 854, the surfaces of metal layers or foils, e.g. copper layers or foils are provided with a coating of a chemical compound consisting of the metal, e.g. copper and a reactive gas, preferably oxygen. This coating forms an eutectic or melting layer together with a thin layer of the adjoining metal, with a melting temperature below the melting temperature of the metal, e.g. copper, so that by applying the metal layer or foil to the ceramic and heating all layers, they are bonded together, namely by melting of the metal essentially only in the area of the melting layer or oxide layer. When copper or a copper alloy is used as the metal, this method is also referred to as DCB bonding or the DCB process (direct copper bonding). This DCB process then comprises, for example, the following process steps:                Oxidation of a copper foil so as to produce an even copper oxide layer;        placing the copper foil on the ceramic layer;        heating the composite to a process temperature between approx. 1025 and 1083° C., e.g. to approx. 1071° C.;        cooling to room temperature.        
Also known is the so-called active soldering method (DE 22 13 115; EP-A-153 618) for bonding metal layers or metal foils forming metallizations, in particular also of copper layers or copper foils, with the respective ceramic material. In this process, which is used especially for manufacturing a metal-ceramic substrate, a bond is produced at a temperature of ca. 800-1000° C. between a metal foil, for example copper foil, and a ceramic substrate, for example aluminum nitride ceramic, using a hard solder, which in addition to a main component such as copper, silver and/or gold also contains an active metal. This active metal, which is at least one element of the group Hf, Ti, Zr, Nb, Ce, creates a bond between the solder and the ceramic through chemical reaction, while the bond between the solder and the metal is a metallic hard solder bond.
Also known is a metal-ceramic substrate with an inner layer or base layer of a silicon nitride ceramic (EP 798 781) that has a significantly higher mechanical strength as compared with other ceramics, in particular as compared with an aluminum oxide ceramic (Al3O2). To enable application of the metallizations with the DCB process, it has been suggested to apply an intermediate layer of a pure aluminum oxide ceramic to the base layer of the silicon nitride ceramic. However, this method does not result in a complete bond, and in particular not in a bond without defects or defective spots, between the ceramic material and the metallization. Rather, especially with the use of metallizations of copper, this method results in numerous gas cavities between the metallization and the ceramic material, due to a reaction between the oxygen and the copper or copper oxide eutectic (CU/Cu2O eutectic) and the silicon nitride ceramic, namely corresponding to the following formula:6CuO+Si3N4→3SiO2+6Cu+N2.
This reaction not only uses up the liquid eutectic Cu/Cu2 phase required for the bonding, but also results in the formation of bubbles due to the gaseous nitrogen (N2). This disadvantageous reaction cannot be avoided by the intermediate layer of pure aluminum oxide ceramic. Based on a finding of the present invention, this is due to the very different thermal expansion coefficients of silicon nitride (3.0×10−6 K−1) and aluminum oxide (8×10−6 K−1). These differences in the thermal expansion coefficient causes cracks in the intermediate layer during melting on or sintering of the intermediate layer of the aluminum oxide ceramic, but also during bonding of the metallizations during the DCB process, so that these cracks allow the above-described reaction between the Cu/Cu2O eutectic and the silicon nitride ceramic.
Also known (EP 0 499 589) is a method for providing at least one intermediate layer of pure silicon oxide (SiO2) on a ceramic base layer and then applying the metallization by means of the DCB process. This procedure likewise does not produce usable results, since the eutectic melting required for the DCB process reacts with the SiO2 to form liquid Cu2O—SiO2. An intermediate layer of SiO2 therefore cannot be used for applying the metallizations using the DCB process.
It is an object of the invention is to present a metal-ceramic substrate that retains the basic advantages of the silicon nitride ceramic while preventing the aforementioned disadvantages.